Machining fixture production

ABSTRACT

Disclosed is a method of producing a machining fixture for fixedly holding a workpiece during machining of that workpiece. The method comprises: providing an initial machining fixture comprising a plurality of receiving elements for receiving the workpiece to be machined; determining a datum, the datum being dependent upon the relative positions of the receiving elements; determining positions and orientations of one or more reference surfaces with respect to the datum; measuring the surface of the initial machining fixture with respect to the datum; and, thereafter, controlling machining apparatus with respect to the datum to machine the initial machining fixture to form the one or more reference surfaces, thereby producing the machining fixture.

RELATED APPLICATIONS

This application is a national phase application filed under 35 USC §371 of PCT Application No. PCT/GB2015/051092 with an Internationalfiling date of Apr. 10, 2015 which claims priority of GB PatentApplication 1407186.4 filed Apr. 24, 2014 and EP Patent Application14275090.0 filed Apr. 24, 2014. Each of these applications is hereinincorporated by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to machining fixtures and the productionthereof.

BACKGROUND

Aircraft airframes typically comprise a plurality of frames (or formers)and longerons (or stringers/stiffeners). The frames are typicallylaterally spaced from one another and arranged perpendicular to thelongitudinal axis of the aircraft. The primary purpose of formers is toestablish the shape of the fuselage and reduce the column length of thelongerons. The longerons are typically elongate members which areattached to the frames and are arranged parallel to the longitudinalaxis of the aircraft. The longerons support the aircraft skin and, inuse, transfer aerodynamic loads acting on the skin onto the frames.

It is desirable that aircraft airframes are produced to be within verytight tolerance bounds.

Production of an aircraft airframe typically comprises producing two ormore separate airframe sections (for example, a fore fuselage section,an aft fuselage section, and a tail section), and subsequently attachingthose sections together.

It tends to be very difficult to produce separate airframe sections witha sufficient level of precision to allow for easy assembly of theairframe. Lengthy and expensive shimming processes may be required tofill gaps between the airframe sections when those sections are attachedtogether.

Production of a section of an aircraft airframe typically involves theuse of airframe assembly tools designed to support airframe componentswhile they are being worked on and to locate different componentstogether in the correct relative positions during airframe assembly.Traditionally, each different assembly process has required at least onededicated assembly tool, which is produced specifically for a given setof components and which is designed to support the components in aparticular manner so that assembly operations can be carried out withoutinterference from the tool. Such assembly tools are manufactured toexacting standards.

A conventional assembly tool comprises a rigid metal jig whose frameworkis constructed from welded box section steel. A plurality of pick-updevices is mounted on the framework for carrying the aircraft componentsduring the assembly process, and these too are conventionally producedfrom welded steel parts.

EP 1 230 124 discloses such an assembly tool.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a method of producinga machining fixture for fixedly holding a workpiece during machining ofthat workpiece, the method comprising: providing an initial machiningfixture comprising a plurality of receiving elements for receiving theworkpiece to be machined; determining a datum, the datum being dependentupon the relative positions of the receiving elements; determiningpositions and orientations of one or more reference surfaces withrespect to the datum; measuring the surface of the initial machiningfixture with respect to the datum; and, thereafter, controllingmachining apparatus with respect to the datum to machine the initialmachining fixture to form the one or more reference surfaces, therebyproducing the machining fixture.

The machining fixture may be an airframe component machining fixture forfixedly holding a workpiece during a process of producing an airframecomponent from that workpiece.

The method may further comprise providing a first digital model of anobject to be produced from the workpiece using the machining fixture.The step of determining positions and orientations of one or morereference surfaces may comprise, using the first digital model,identifying one or more features of the object, and, for each identifiedfeature, determining a position and orientation of a reference surfacerelative to the datum.

The object to be produced may be an airframe component. Each objectfeature may be a landing for receiving a further airframe componentduring an airframe assembly process.

The method may further comprise, using the first digital model and thedetermined positions and orientations of the one or more referencesurfaces, determining a second digital model, the second digital modelbeing a model of the machining fixture specifying the positions andorientations of the reference surfaces. The step of controllingmachining apparatus may be performed using the second digital model.

The step of determining the datum may comprise measuring the relativepositions of the receiving elements on the initial machining fixture,and, using the measurements of the relative positions of the receivingelements, calculating the datum.

The step of determining the datum may comprise: providing the workpieceto be machined, the workpiece comprising a plurality of attachmentfeatures for attaching to the receiving elements of the machiningfixture, the relative positions of the attachment features beingdependent upon the relative positions of the receiving elements;measuring the relative positions of the attachment features on theworkpiece; and, using the measurements of the relative positions of theattachment features, calculating the datum.

In a further aspect, the present invention provides a machining processcomprising: producing a machining fixture using a method according toany of the preceding aspects; providing a workpiece to be machined, theworkpiece comprising a plurality of attachment features; attaching theworkpiece to the machining fixture by attaching the receiving elementsof the machining fixture to the attachment features of the workpiece;and, thereafter, performing, using machining apparatus, a machiningoperation to machine the workpiece so as to produce a predeterminedobject.

The object may be an airframe component selected from the group ofairframe components consisting of a frame and a longeron.

Each reference surface of the machining fixture may be associated withan object feature of the object that is to be produced. Machining theworkpiece may comprise, for each reference surface, locating at leastpart of the machining apparatus against that reference surface, andcontrolling the machining apparatus to move away from that referencesurface with respect to the datum to machine the workpiece so as to formthe object feature associated with that reference surface.

Each object feature may be a landing for receiving a further airframecomponent during an airframe assembly process.

In a further aspect, the present invention provides apparatus forproducing a machining fixture for fixedly holding a workpiece duringmachining of that workpiece, the method comprising: means for providingan initial machining fixture comprising a plurality of receivingelements for receiving the workpiece to be machined; one or moreprocessors configured to determine a datum, the datum being dependentupon the relative positions of the receiving elements, and determinepositions and orientations of one or more reference surfaces withrespect to the datum; measuring apparatus configured to measure thesurface of the initial machining fixture with respect to the datum; andmachining apparatus configured to be controlled with respect to thedatum so as to machine the initial machining fixture to form the one ormore reference surfaces, thereby producing the machining fixture.

In a further aspect, the present invention provides a machining fixtureproduced using a method according to any of the preceding aspects.

The machining fixture may comprise a rigid base portion, and thereceiving elements may extend from an upper surface of the base portion.

The reference surfaces may be formed from sacrificial blocks attached tothe base portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration (not to scale) of an exampleaircraft;

FIG. 2 is a schematic illustration (not to scale) of a portion of anairframe of an aft fuselage of the aircraft;

FIG. 3 is a schematic illustration (not to scale) showing a side view ofa first frame;

FIG. 4 is a process flow chart showing certain steps of a process ofproducing the first frame;

FIG. 5 is a schematic illustration (not to scale) of a perspective viewof a frame machining fixture;

FIG. 6 is a schematic illustration (not to scale) showing a top-downview of a workpiece coupled to the frame machining fixture;

FIG. 7 is a process flow chart showing certain steps of an embodiment ofa process of producing the airframe;

FIG. 8 is a schematic illustration (not to scale) showing a perspectiveview of an embodiment of an assembly jig;

FIG. 9 is a schematic illustration (not to scale) showing packermaterial applied to an outer surface of the components of the airframe;

FIG. 10 is a schematic illustration (not to scale) showing an exampleway in which the packer material may be applied to a surface of anairframe component;

FIG. 11 is a schematic illustration (not to scale) showing a furtherexample way in which the packer material may be applied to a surface ofan airframe component; and

FIG. 12 is a process flow chart showing certain steps of an embodimentof a process of producing the jig.

DETAILED DESCRIPTION

It will be appreciated that relative terms such as horizontal andvertical, top and bottom, above and below, front and back, upper andlower, and so on, are used herein merely for ease of reference to theFigures, and these terms are not limiting as such, and any two differingdirections or positions and so on may be implemented rather than trulyhorizontal and vertical, top and bottom, and so on.

FIG. 1 is a schematic illustration (not to scale) of an example aircraft2 comprising an aft fuselage 4, and an aircraft fore section 5. Theaircraft fore section 5 includes a central fuselage to which the aftfuselage 4 is attached.

The aft fuselage 4 comprises an airframe and an aircraft skin fastenedto the airframe. In this embodiment, the aft fuselage 4 comprises a portboom and a starboard boom that are mechanically attached together alonga centreline. Each boom of the aft fuselage 4 comprises a plurality ofstructural components (for example, frames, keels, longerons, and skins)that are mechanically attached together.

FIG. 2 is a schematic illustration (not to scale) of a portion of theairframe 6 of the aft fuselage 4. In this embodiment, the portion of theairframe 6 is at least a part of a boom of the aft fuselage 4 (i.e.either a port or starboard boom that is to be attached to an oppositeboom).

In this embodiment, the airframe 6 comprises three laterallyspaced-apart frames or “formers”, namely a first frame 8, a second frame10, and a third frame 12; three longerons, namely a first longeron 14, asecond longeron 16, and a third longeron 18; and a beam 20. For reasonsof clarity, the longerons 14, 16, 18 and the beam 20 are shown as hashedin FIG. 2.

In this embodiment, the frames 8, 10, 12 are made of aluminium ortitanium. The frames 8, 10, 12 define the shape of the aircraft fuselageand, in use, provide stability to the aircraft 2 by preventing oropposing deflection of the longerons 14, 16, 18. When the aircraft 2 isfully assembled, the frames 8, 10, 12 are arranged substantiallyperpendicularly to the longitudinal axis of the aircraft 2

In this embodiment, the longerons 14, 16, 18 are made of aluminium ortitanium. The longerons 14, 16, 18 are elongate members to which theskin of the aircraft is fastened. When the aircraft 2 is fullyassembled, the longerons 14, 16, 18 run substantially parallel to thelongitudinal axis of the aircraft 2. In this embodiment, the longerons14, 16, 18 are fastened to the frames 8, 10, 12 by a plurality of bolts.In this embodiment, the first longeron 14 has a first end 14 a attachedto the first frame 8 and a second free end 14 b proximate to the thirdframe 12. Similarly, the second longeron 16 has a first end 16 aattached to the first frame 8 and a second free end 16 b proximate tothe third frame 12. Similarly, the third longeron 18 has a first end 18a attached to the first frame 8 and a second free end 18 b proximate tothe third frame 12.

In this embodiment, during assembly of the aircraft 2, the second freeends 14 b, 16 b, 18 b of the longerons 14, 16, 18 are attached to thefore fuselage 5 of the aircraft 2. In particular, the second free ends14 b, 16 b, 18 b of the longerons 14, 16, 18 are attached to a frame ofthe fore fuselage 5. This frame of the fore fuselage 5 to which thesecond free ends 14 b, 16 b, 18 b of the longerons 14, 16, 18 are to beattached is hereinafter referred to as the “fore fuselage frame”.

In this embodiment, the beam 20 is made of aluminium or titanium. Thebeam 20 is an elongate member. The beam 20 is attached at its first end(i.e. a proximal end) to the first frame 8 by a plurality of bolts, andextends away from the frames 8, 10, 12 to its second end (i.e. a distalfree end) opposite to its first end. When the aircraft 2 is fullyassembled, the beam 20 runs substantially parallel to the longitudinalaxis of the aircraft 2.

In this embodiment, in addition to being attached together by thelongerons 14, 16, 18, the frames 8, 10, 12 are attached together byfurther structural elements often called “keels” which are locatedbetween adjacent frames 8, 10, 12. For ease of illustration and clarity,these keels are not shown in FIG. 2.

In this embodiment, to produce the aft fuselage 4, a composite aircraftskin is fastened to the airframe 6. The outer shape of the assembled aftfuselage 4 (i.e. the outer shape of the aft fuselage 4 produced byfastening the composite skin to the airframe 6) is hereinafter referredto as the Outer Mould Line (OML) of the aft fuselage 4. In thisembodiment, the OML of the aft fuselage 4 is to be within apre-specified tolerance. The OML of the aft fuselage 4 having therequired tolerance is facilitated by the Inner Mould Line (IML) of theaft fuselage 4 being within a pre-specified tolerance. The IML of theaft fuselage 4 is the surface at which the airframe 6 and the aircraftskin abut, i.e. an outer surface of the airframe 6 and inner surface ofthe aircraft skin.

An embodiment of a process of producing the airframe 6 is described inmore detail later below with reference to FIG. 7.

FIG. 3 is a schematic illustration (not to scale) showing a side view ofthe first frame 8.

The first frame 8 comprises a plurality of longeron landings 30 a-c towhich, during assembly of the airframe 4, the longerons 14, 16, 18 arefastened. In particular, the first frame 8 comprises a first longeronlanding 30 a shaped to receive a portion of the first longeron 14, asecond longeron landing 30 b shaped to receive a portion of the secondlongeron 16, and a third longeron landing 30 c shaped to receive aportion of the third longeron 18. The longeron landings 30 a-c areattachment features to which other components, in particular thelongerons 14-18, attach.

The first frame 8 further comprises a plurality of fixture attachmentfeatures 32. In this embodiment, there are four fixture attachmentfeatures 32. As described in more detail later below, the fixtureattachment features 32 are for attaching the first frame 8 to a fixturesuch as a machining fixture and/or an assembly fixture. In thisembodiment, the fixture attachment features 32 are holes through thestructure of the first frame 8 through which locator pins or otherelongate members may be positioned.

In this embodiment, similarly to the first frame 8, the second frame 10also comprises three longeron landings, each longeron landing beingconfigured to receive a portion of a respective longeron 14, 16, 18.Also, the second frame 10 comprises a plurality of fixture attachmentfeatures.

In this embodiment, similarly to the first and second frames 8, 10, thethird frame 12 also comprises three longeron landings, each longeronlanding being configured to receive a portion of a respective longeron14, 16, 18. Also, the third frame 12 comprises a plurality of fixtureattachment features.

FIG. 4 is a process flow chart showing certain steps of a process ofproducing the first frame 8. A similar process, mutatis mutandis, may beused to produce the second and third frames 10, 12. A similar process,mutatis mutandis, may be used to produce the beam 20. A similar process,mutatis mutandis, may be used to produce the longerons 14, 16, 18.

At step s2, a human designer generates or creates a digital model,hereinafter referred to as the “first digital model”. The first digitalmodel is of the first frame 8. This may be performed using a computerand an appropriate software package, for example, the Catia (Trademark)V4 software package. The first digital model may defined using a digitalmodel of the airframe 6 or aft fuselage 4.

At step s4, the human designer generates or creates a further digitalmodel, hereinafter referred to as the “second digital model”. The seconddigital model is of a frame machining fixture. The frame machiningfixture is a fixture system that is to be used to secure a workpiece inplace whilst that workpiece is machined to form the first frame 8. Theframe machining fixture will be described in more detail later belowwith reference to FIG. 5.

At step s6, a forging system produces a first forging. The first forgingis an aluminium blank from which the first frame 8 is to be produced. Adigital model of the first forging may be used to produce the firstforging. This digital model may be defined using a digital model of theairframe 6 or aft fuselage 4.

At step s8, using the first digital model, a 5-axis computer numericalcontrol (CNC) milling machine machines the first forging to produce aworkpiece from which the first frame 8 is to be produced.

In this embodiment, the workpiece is essentially the same shape as thefirst frame 8 except that the workpiece comprises additional materialwhere the longeron landings 30 a-c are to be located. The workpiececomprises the fixture attachment features 32. The workpiece will bedescribed in more detail later below with reference to FIG. 5.

At step s10, the forging system produces a second forging. The secondforging is a steel blank from which the frame machining fixture is to beproduced. In other embodiments, the blank from which the frame machiningfixture is to be produced is a structure created by bolting or welding aplurality of substructures together. A digital model of the secondforging may be used to produce the second forging. This digital modelmay be defined using a digital model of the airframe 6 or aft fuselage4.

At step s12, using the second digital model, the 5-axis CNC millingmachine machines the second forging so as to produce the frame machiningfixture. In other embodiments, the frame machining fixture is created bybolting or welding a plurality of substructures together. The firstdigital model may also be used in the production of the frame machiningfixture.

FIG. 5 is a schematic illustration (not to scale) of a perspective viewof the frame machining fixture 40.

In this embodiment, the frame machining fixture 40 comprises asubstantially rigid base portion 41, a plurality of locator pins 42, anda plurality of precision ground blocks 44 a-c.

In this embodiment, there are four locator pins 42. The locator pins 42are located on an upper surface of the base portion 41 and extend awayfrom the upper surface of the base portion 41 in a direction that issubstantially perpendicular to that surface. In this embodiment, each ofthe locator pins 42 is configured to couple to a respective fixtureattachment feature 32 of the workpiece. The relative positions of thelocator pins 42 correspond to those of the fixture attachment features32 such that the workpiece may be placed onto the upper surface of thebase portion 41 in such a way that each locator pin 42 couples to arespective fixture attachment feature 32, thereby securing the workpieceagainst the frame machining fixture 40. The workpiece and the framemachining fixture 40 are complementary. The locator pins 42 areconfigured to securely hold the workpiece to prevent or oppose movementor deflection of the workpiece while the workpiece is being machined.

In this embodiment, the first and second digital models are createdconcurrently. This tends to facilitate in the production of thecomplementary workpiece and the frame machining fixture 40.

In this embodiment, there are three blocks 44 a-c, namely a first block44 a, a second block 44 b, and a third block 44 c. The blocks 44 a-c arelocated on an upper surface of the base portion 41 and extend away fromthe upper surface of the base portion 41 in a direction that issubstantially perpendicular to the upper surface of the base portion 41.In this embodiment, the locations of the blocks 44 a-c are such that,when the workpiece is coupled to the frame machining fixture 40 bylocating the locator pins 42 in the fixture attachment features 32, eachblock 44 a-c is proximate to a respective surface of the workpiece thatis to be machined so as to form a longeron landing 30 a-c. Furthermore,each block 44 a-c includes a surface that is substantially parallel witha respective surface of the workpiece that is to be machined so as toform a longeron landing 30 a-c. In particular, the first block 44 a isproximate to and substantially parallel with the surface of theworkpiece that is to be machined so as to form the first longeronlanding 30 a. Similarly, the second block 44 b is proximate to andsubstantially parallel with the surface of the workpiece that is to bemachined so as to form the second longeron landing 30 b. Similarly, thethird block 44 c is proximate to and substantially parallel with thesurface of the workpiece that is to be machined so as to form the thirdlongeron landing 30 c.

The positions of the blocks 44 a-c in the second digital model may bedetermined using the positions of the longeron landings 30 a-c in thefirst digital model.

In some embodiments, a coordinate measuring machine (CMM) is used toinspect the locator pins 42 and/or the blocks 44 a-c. The framemachining fixture 40 may be further processed, i.e. adjusted, to ensurethat the frame machining fixture 40 is as specified by the seconddigital model, for example, the locator pins 42 and/or the blocks 44 a-cmay be further machined dependent upon the CMM measurements. Thisprocesses of measuring and adjusting the machining fixture may beperformed iteratively.

At step s14, a coordinate measuring machine (CMM) measures the relativelocations of the fixture attachment features 32 of the workpieceproduced at step s8.

At step s16, using the CMM measurements of the fixture attachmentfeatures 32 on the workpiece, a computer determines a datum, hereinafterreferred to as the “frame datum”. The frame datum is a reference system,with respect to the attachment features, from which measurements may bemade. The frame datum may be computed using any appropriate softwarepackage, for example, the Valisys (Trademark) software package.

At step s18, the CMM measures the surface of the workpiece with respectto the frame datum. Thus, the locations of points on the surface of theworkpiece with respect to the fixture attachment features 32 aredetermined.

At step s20, the workpiece is fixedly secured to the frame machiningfixture 40 by placing the workpiece onto the upper surface of the baseportion 41 such that each locator pin 42 is coupled to (i.e. positionedthrough) a respective fixture attachment feature 32. The locator pins 42may be threaded and the workpiece may be secured to the threaded locatorpins 42 by screwing nuts onto the threaded pins.

FIG. 6 is a schematic illustration (not to scale) showing a top-downview of the workpiece 50 coupled to the frame machining fixture 40.

The three portions of the workpiece 50 that are to be machined so as toproduce the longeron landings 30 a-c are hereinafter referred to as the“excess portions” and are indicated in FIG. 6 by hashed regions and thereference numerals 52 a-c. As shown in FIG. 6, the blocks 44 a-c providesurfaces that are proximate to and substantially parallel with thesurfaces of the workpiece 50 that are to be machined to form thelongeron landings 30 a-c.

At step s22, while the workpiece 50 is attached to the frame machiningfixture 40, the 5-axis CNC milling machine machines the workpiece so asto remove the excess portions 52 a-c, thereby forming the longeronlandings 30 a-c and producing the first frame.

In this embodiment, the first excess portion 52 a of the workpiece 50 isremoved/machined so as to form the first longeron landing 30 a. Also,the second excess portion 52 b of the workpiece 50 is removed/machinedso as to form the second longeron landing 30 b. Also, the third excessportion 52 c of the workpiece 50 is removed/machined so as to form thethird longeron landing 30 c.

In this embodiment, the removal of the first excess portion 52 a isperformed as follows.

Firstly, the 5-axis CNC milling machine probes the surface of the firstblock 44 a, for example, by moving so as to contact with the surface ofthe first block 44 a that is parallel with the surface of the workpiece50 to be machined. In this way, the CNC milling machine determines thelocation of its cutting tool in space with respect to the frame datum.

In this embodiment, the positional relationship between the blocks 44a-c and the locator pins 42 is known from the second digital model.Also, when the workpiece 50 is coupled to the frame machining fixture40, the locator pins 42 are substantially collocated with the fixtureattachment features 32 of the workpiece 50. Thus, the positions of theblocks 40 a-c with respect to the frame datum are known. Thus, when CNCmachine probes (i.e. contacts with) the first block 44 a, the positionof the cutting tool with respect to the frame datum is known/determinedwith relatively high accuracy.

Secondly, using the known position of the cutting tool, the measurementsof the surface of the workpiece taken at step s18, and the first digitalmodel (which specifies the shape of the first longeron landing 30 a),the CNC machine machines away the first excess portion 52 a of theworkpiece 50 so as to form the first longeron landing 30 a. In thisembodiment, this machining is performed by moving the cutting tool withrespect to the frame datum such that the first longeron landing 30 a isformed with a high degree of accuracy relative to the frame datum, i.e.the fixture attachment features 32. Preferably, the cutting tool ismoved from the surface of the first block 44 a that is parallel with thesurface of the workpiece 50 to be machined, in only a single direction,i.e. along only a single axis. This limited movement of the cutting toolfrom the block advantageously tends to reduce errors.

The removal of the second and third excess portions 52 b, 52 c to formthe second and third longeron landings 30 b, 30 c respectively isperformed using an analogous method to that described above for theforming of the first longeron landing 30 a. Thus, in this embodiment,the longeron landings 30 a-c are formed with high accuracy with respectto the frame datum (i.e. the fixture attachment features 32).

Thus, a process of producing the first frame 8 is provided.

In some embodiments, measurements of the positions of the blocks 44 a-crelative to the locator pins 42 may be used to determine the positionsof the blocks 44 a-c in the frame datum.

In some embodiments, measurements of the surface of the frame machiningfixture 40 (e.g. taken using a CMM) are used to create the “framedatum”. For example, the frame datum may be created using measurementsof the locator pins 42 on the frame machining fixture 40.

An embodiment of a process of producing the airframe 6 described in moredetail above with reference to FIG. 2 will now be described.

FIG. 7 is a process flow chart showing certain steps of an embodiment ofa process of producing the airframe 6.

At step s32, an assembly jig is produced. A process of producing the jigis described in more detail later below with reference to FIG. 12.

FIG. 8 is a schematic illustration (not to scale) showing a perspectiveview of an embodiment of the jig 60 produced at step s32.

In this embodiment, the jig 60 comprises a steel jig frame 62 comprisinga plurality of steel beams that are attached together (e.g. by welding)to form a rectangular parallelepiped.

The jig 60 further comprises a steel reference frame 64 attached (e.g.by welding) to one end of the jig frame 62. In this embodiment, thereference frame 64 is, in effect, a copy of the fore fuselage frame towhich, during assembly of the aircraft 2, the aft fuselage 4 is to beattached. In other words, the reference frame 64 is substantially thesame as the frame of the fore fuselage 5 to which the second free ends14 b, 16 b, 18 b of the longerons 14, 16, 18 are to be attached.

The jig 60 further comprises four sets of pickup devices, hereinafterreferred to as “jig pickups”. Preferably, the jig pickups are ofuniversal construction. In this embodiment, each jig pickup comprises amounting element for mounting the device on the jig frame 62, areceiving element for carrying an airframe component, and a plurality ofelongate members having predetermined lengths connected together bymeans of clamping elements so as to allow six degrees of freedom ofmovement of the receiving element around three orthogonal axes. Examplesof appropriate jig pickups include, but are not limited to, thosedescribed in EP1230124 and EP1600379, each of which is incorporated inits entirety herein by reference. The pick-up devices may be formed fromaluminium.

Each of the first set of jig pickups is indicated in FIG. 8 by thereference numeral 66. Each of the second set of jig pickups is indicatedin FIG. 8 by the reference numeral 68. Each of the third set of jigpickups is indicated in FIG. 8 by the reference numeral 70. Each of thefourth set of jig pickups is indicated in FIG. 8 by the referencenumeral 72.

As described in more detail later below with reference to FIG. 12, thefirst jig pickups 66 are configured to securely hold the first frame 8in a predetermined position relative to the reference frame 64.

In this embodiment, each of the first jig pickups 66 is attached to thejig frame 62, e.g. by bolts or welding. Attachment using boltsadvantageously tends to permit adjustment of the first jig pickups 66 sothat their positions on the jig frame 62 are in accordance with adigital model that specifies those positions, and also allows for theremoval of the first jig pickups 66 from the jig frame 62. Each of thefirst jig pickups 66 comprises an elongate arm to which is attached areceiving element 74, hereinafter referred to as a “first receivingelement”. Each of the first receiving elements 74 is configured tocouple to a respective fixture attachment feature 32 of the first frame8.

For ease of illustration, only two first jig pickups 66 are shown inFIG. 8. However, in reality, the number of first jig pickups 66 is suchthat the number of first receiving elements 74 is equal to the number offixture attachment features 32 of the first frame 8.

As described in more detail later below with reference to FIG. 12, thesecond jig pickups 68 are configured to securely hold the second frame10 in a predetermined position relative to the reference frame 64.

In this embodiment, each of the second jig pickups 68 is attached to thejig frame 62, e.g. by bolts or welding. Each of the second jig pickups68 comprises an elongate arm to which is attached a receiving element76, hereinafter referred to as a “second receiving element”. Each of thesecond receiving elements 76 is configured to couple to a respectiveattachment feature of the second frame 10.

For ease of illustration, only two second jig pickups 68 are shown inFIG. 8. However, in reality, the number of second jig pickups 68 is suchthat the number of second receiving elements 76 is equal to the numberof attachment features of the second frame 10.

As described in more detail later below with reference to FIG. 12, thethird jig pickups 70 are configured to securely hold the third frame 12in a predetermined position relative to the reference frame 64.

In this embodiment, each of the third jig pickups 70 is attached to thejig frame 62, e.g. by bolts or welding. Each of the third jig pickups 70comprises an elongate arm to which is attached a receiving element 78,hereinafter referred to as a “third receiving element”. Each of thethird receiving elements 78 is configured to couple to a respectiveattachment feature of the third frame 12.

For ease of illustration, only two third jig pickups 70 are shown inFIG. 8. However, in reality, the number of third jig pickups 70 is suchthat the number of third receiving elements 78 is equal to the number ofattachment features of the third frame 12.

As described in more detail later below with reference to FIG. 12, thefourth jig pickups 72 are configured to securely hold the second ends ofthe beams 20, 22 in a predetermined position relative to the referenceframe 64.

In this embodiment, each of the fourth jig pickups 72 is attached to thejig frame 62, e.g. by bolts or welding. Each of the fourth jig pickups72 comprises an elongate arm to which is attached a receiving element80, hereinafter referred to as a “fourth receiving element”. Each of thefourth receiving elements 80 is configured to couple to a respectiveattachment feature of a beam 20, 22.

For ease of illustration, only two fourth jig pickups 72 are shown inFIG. 8. However, in reality, the number of fourth jig pickups 72 is suchthat the number of fourth receiving elements 80 is equal to the numberof attachment features of the beams 20, 22.

At step s34, each of the frames 8, 10, 12 are produced as described inmore detail earlier above with reference to FIG. 4.

At step s36, the frames 8, 10, 12 are attached to the jig 60.

In particular, in this embodiment the first frame 8 is attached to thefirst jig pickups 66 such that each of the first receiving elements 74is coupled to a respective fixture attachment feature 32 of the firstframe 8. Thus, the first frame 8 is fixedly attached to the jig 60 at apredetermined position relative to the reference frame 64. In thisembodiment, as described in more detail later below with reference toFIG. 12, the position on the jig 60 of the first frame 8 with respect tothe reference frame 64 is the same as the desired position on theassembled aircraft 2 of the first frame 8 with respect to the forefuselage frame.

Also, in this embodiment the second frame 10 is attached to the secondjig pickups 68 such that each of the second receiving elements 76 iscoupled to a respective attachment feature of the second frame 10. Thus,the second frame 10 is fixedly attached to the jig 60 at a predeterminedposition relative to the reference frame 64. In this embodiment, asdescribed in more detail later below with reference to FIG. 12, theposition on the jig 60 of the second frame 10 with respect to thereference frame 64 is the same as the desired position on the assembledaircraft 2 of the second frame 10 with respect to the fore fuselageframe.

Also, in this embodiment the third frame 12 is attached to the third jigpickups 70 such that each of the third receiving elements 78 is coupledto a respective attachment feature of the third frame 12. Thus, thethird frame 12 is fixedly attached to the jig 60 at a predeterminedposition relative to the reference frame 64. In this embodiment, asdescribed in more detail later below with reference to FIG. 12, theposition on the jig 60 of the third frame 12 with respect to thereference frame 64 is the same as the desired position on the assembledaircraft 2 of the third frame 12 with respect to the fore fuselageframe.

At step s38, the beam 20 is attached to the jig 60 and the first frame8.

In this embodiment, the beam 20 comprises one or more attachmentfeatures, i.e. through holes, proximate to its second end. In thisembodiment, the beam 20 is attached to the fourth jig pickups 72 suchthat each of the fourth receiving elements 80 is coupled to a respectiveattachment feature of the beam 20. Also, the first end of the beam 20 isattached, e.g. by bolts, to the first frame 8. Thus, the beam 20 isfixedly attached to the jig 60 and the first frame 8. In thisembodiment, the beam 20 is attached to the jig 60 such that the secondend of the beam 20 is at a predetermined position relative to thereference frame 64. In this embodiment, as described in more detaillater below with reference to FIG. 12, the position on the jig 60 of thesecond end of the beam 20 with respect to the reference frame 64 is thesame as the desired position on the assembled aircraft 2 of the secondend of the beam 20 with respect to the fore fuselage frame.

At step s40, the keels are attached between adjacent frames 8, 10, 12e.g. by bolting. In this embodiment, the keels not directly attached tothe jig 60 e.g. by pickups, and instead are attached to the frames 8,10, 12 only.

At step s42, the longerons 14, 16, 18 are attached to the jig 60 and thefirst frame 8.

In particular, in this embodiment, the first longeron 14 is attached tothe first longeron landing 30 a of the first frame 8, a first longeronlanding of the second frame 10, and a first longeron landing of thethird frame 12. Also, the second free end 14 b of the first longeron 14may be attached to a feature of the reference frame 64 that correspondsto a feature of the fore fuselage frame to which, during assembly of theaircraft 2, the second free end 14 b of the first longeron 14 is to beattached. In some embodiments, the first free end 14 a of the firstlongeron 14 may be attached to the first frame 8 and/or the jig 60 e.g.to a jig pickup.

Also, in this embodiment, the second longeron 16 is attached to thesecond longeron landing 30 b of the first frame 8, a second longeronlanding of the second frame 10, and a second longeron landing of thethird frame 12. Also, the second free end 16 b of the second longeron 16may be attached to a feature of the reference frame 64 that correspondsto a feature of the fore fuselage frame to which, during assembly of theaircraft 2, the second free end 16 b of the second longeron 16 is to beattached. In some embodiments, the first free end 16 a of the secondlongeron 16 may be attached to the first frame 8 and/or the jig 60 e.g.to a jig pickup.

Also, in this embodiment, the third longeron 18 is attached to the thirdlongeron landing 30 c of the first frame 8, a third longeron landing ofthe second frame 10, and a third longeron landing of the third frame 12.Also, the second free end 18 b of the third longeron 18 may be attachedto a feature of the reference frame 64 that corresponds to a feature ofthe fore fuselage frame to which, during assembly of the aircraft 2, thesecond free end 18 b of the third longeron 18 is to be attached. In someembodiments, the first free end 18 a of the third longeron 18 may beattached to the first frame 8 and/or the jig 60 e.g. to a jig pickup.

As the longeron landings of the frames 8, 10, 12 have been machined withhigh accuracy with respect to the fixture attachment features 32 of theframes 8, 10, 12 (i.e. to the frame datums), and the frames 8, 10, 12have been located on jig 60 at predetermined positions with respect tothe reference frame 64 using the fixture attachment features 32 of theframes 8, 10, 12, when the longerons 14, 16, 18 are attached to thelongeron landings, the longerons 14, 16, 18 tend to be accuratelylocated at predetermined positions with respect to the reference frame64. In this embodiment, the positions on the jig 60 of the longerons 14,16, 18 with respect to the reference frame 64 is the same as the desiredposition on the assembled aircraft 2 of the third frame 12 with respectto the fore fuselage frame.

A step s44, sacrificial plies or layers of material, which arecollectively referred to hereinafter as “packer material” are applied,e.g. using an adhesive, to the surfaces of the airframe components (i.e.the surfaces of the frames 8, 10, 12, the longerons 14, 16, 18, and thebeam 20) to which the aircraft skin is to be attached.

In this embodiment, the packer material is applied to the airframecomponents after the airframe 6 has been assembled in the jig 60.However, in other embodiments, the packer material is applied to theairframe components prior to the airframe 6 being assembled in the jig60.

Steps s46 to s50 of the process of FIG. 7 will be described in moredetail later below after a description of the packer material and itsapplication to the airframe components.

FIG. 9 is a schematic illustration showing packer material 90 applied toa surface of the first frame 8 and the first longeron 14.

The desired IML for the airframe 6 is shown in FIG. 9 as a dotted lineand is indicated by the reference numeral 92. The IML 92 may bespecified in the digital model of the airframe 6 or aft fuselage 4.

In this embodiment, the packer material 90 is made of a compositematerial such as a fibre-reinforced polymer, i.e. a polymer matrixreinforced with fibres (e.g. carbon or glass fibres). The packermaterial 90 is a different material to that/those from which theairframe components are made

In this embodiment, the packer material 90 is of sufficient thickness toprovide that desired IML 92 is at or beneath the outer surface of thepacker material 90. The thickness for the packer material may bedetermined by performing a tolerance analysis using a detail stageinspection of the components and determined assembly tolerances. In someembodiments, the thickness of the packer material is 1.75 mm to 2.29 mm,e.g. 2.03 mm.

In some embodiments, packer material 90 is not added to some or all ofthe surfaces of the airframe components. For example, in someembodiments, if the outer surface of an airframe component is located atthe desired IML 92, packer material 90 is not added to that surface.

The application of the packer material 90 to the components of theairframe 6 may be performed in any appropriate way. In some embodiments,the packer material 90 comprises flat sheets which are pressed againstand attached to the components of the airframe 6. Alternatively, thepacker material 90 may be shaped to fit against, i.e. be complementaryto, the components of the airframe 6. For example, the packer material90 may be moulded into an appropriate shape by applying the packermaterial 90 to a mould tool and curing the packer material 90 in anautoclave.

FIG. 10 is a schematic illustration showing an example way in which thepacker material 90 may be applied to an outer surface of the first frame8.

In this embodiment, an adhesive 94 is used to adhere the packer material90 to the first frame 8. An over press tool 96 is used to hold thepacker material 90 in place while the adhesive 94 cures or solidifies.

In this embodiment, the over press tool 96 comprises a rigid bodyportion 98 (also referred to as a “strong back”) and a deformableportion 100. The body portion 98 is substantially non deformable. Thebody portion 98 comprises a plurality of clamp receiving elements 102for receiving respective clamp devices 104 such as G-clamps.

In use, the over press tool 96 is positioned onto a portion of theairframe 6 such that packer material 90 is between the over press tool96 and the first frame 8, and such that the deformable portion 100 ofthe over press tool 96 is in contact with the packer material 90. Theover press tool 96 is then attached to and pressed against the packermaterial 90 and first frame 8 using a plurality of clamp devices 104which engage with the clamp receiving elements 102 of the body portion98 and the first frame 8. The over press tool 96 remains in place untilthe adhesive 94 has cured and the packer material 90 is securelyattached to the first frame 8, at which point the clamp devices 104 andthe over press tool 96 are removed.

In this embodiment, the over press tool 96 is shaped to be complementaryto a part of the first frame to which the packer material is to beapplied. The first digital model (i.e. the digital model of the firstframe 8) may be used to specify a shape for the over press tool 96and/or specify a digital model for producing the over press tool 96. Thebody portion 98 may then be produced using an appropriate process, suchas an Additive Layer Manufacturing (ALM) process, from any appropriatematerial, such as plastic.

The deformable portion 100 advantageously tends to provide that theforce exerted by the over press tool 96 onto the packer material 90 issubstantially evenly distributed. Furthermore, the deformable portion100 tends to facilitate the use of the over press tool 96 in cases wherethere are assembly positioning errors etc.

In this example, the over press tool 96 is held against the packermaterial 90 and first frame 8 using a plurality of clamp devices 104.However, in other examples, the over press tool 96 may be held againstthe packer material 90 and an airframe component in a different way.

FIG. 11 is a schematic illustration showing a further example way inwhich the packer material 90 may be applied to an outer surface of thefirst frame 8.

In this embodiment, the over press tool 96 further comprises a rigid arm106 connecting the body portion 98 to an actuation device 108. Theactuation device 108 may, for example be a pneumatic or hydraulicactuation device 108 configured to actuate the over press tool 96. Theover press tool 96 further comprises a vacuum line 110 connecting thesurface of the deformable portion 100 to a vacuum pump 112.

In operation, the first frame 8 is securely held by a support structure114 (e.g. the jig 60). The packer material 90 is held onto thedeformable portion 100 of the over press tool 96 by establishing avacuum in the vacuum line 110. The over press tool 96 is then actuatedby the actuation device 108 in such a way that the packer material 90coupled to the over press tool 96 is brought into contact with the firstframe 8 (i.e. the over press tool 96 is moved in the direction of thearrows in FIG. 11) and firmly pressed against the first frame 8 untilthe adhesive 94 cures. The vacuum holding the packer material 90 to theover press tool 96 may then be released and the over press tool 96 maybe moved away from the first frame 8 leaving the packer material 90adhered thereto.

Having the packer material 90 be retained against the over press tool 96and subsequently bringing the packer material 90 coupled to the overpress tool 96 into contact with the first frame 8 (as opposed toapplying the packer material 90 to the first frame and subsequentlybringing the over press tool into contact with the packer material 90 onthe first frame 8) advantageously tends to reduce unwanted movement ofthe packer material 90 with respect to the first frame 8. For example, alikelihood of the packer material 90 that is applied to the first frame8 moving with respect to the first frame 8 when the over press tool 96is brought into contact with the packer material 90 on the first frame 8tends to be reduced or eliminated. Thus, accuracy with which packermaterial 90 is applied to an object tends to be increased.

Furthermore, having the packer material 90 be retained against the overpress tool 96 and subsequently bringing the packer material 90 coupledto the over press tool 96 into contact with the first frame 8 tends toreduce the likelihood of the packer material 90 adhering to the firstframe 8 before a pressing force is applied. This tends to improveadherence of the packer material to the object.

Returning now to the process of FIG. 7, at step s46, optionally, a lasertracker or CMM is used to measure the outer surface of the packermaterial 90 that has been applied to the components of the airframe 6.In this embodiment, the outer surface of the packer material 90 ismeasured with respect to a so-called “jig datum” which will be describedin more detail later below with reference to FIG. 12. In someembodiments, this step may be omitted or may be performed at a laterstage.

At step s48, a CNC cutting device is used to machine the packer material90 such that the outer surface of the packer material 90 is located atthe IML 92 specified in the digital model of the airframe 6 or aftfuselage 4 with respect to the fore reference frame 64. Optionally, thismay be performed using the laser tracker measurements taken at step s46.In this embodiment, the CNC cutting device is controlled with respect tothe “jig datum” *which is described in more detail later below.

Thus, the packer material 90 is machined to allow for variations incomponent thicknesses and substructure assembly positioning errors. Thistends to provide that, when the aircraft skin is attached to theairframe 6, the OML of the aft fuselage 4 is within the pre-specifiedtolerance with respect to the fore reference frame 4 (i.e. fore fuselagein the assembled aircraft 2). Furthermore, the machined packer material90 provides a consistent landing surface to which the aircraft skin maybe fixed. The landing surface provided by the machined packer material90 is accurately positioned with respect to the jig datum.

Use of the sacrificial packer material 90 advantageously tends to reduceor eliminate a need to machine the surfaces of the airframe components(i.e. the surfaces of the frames 8, 10, 12, the longerons 14, 16, 18,and the beam 20) to which the aircraft skin is to be attached. Thus, aneed for performing post-machining analysis/testing of the airframecomponents may be avoided or reduced.

Use of the packer material 90 advantageously tends to reduce oreliminate a need for shims to be applied to fill gaps between theairframe 6 and the external aircraft skin when the aircraft skin isattached to the airframe 6.

Steps s46 and s48 may be performed iteratively.

Thus, the airframe 6 is assembled using the jig 60.

At step s50, the assembled airframe 6 may be removed from the jig 60.

Thus, an embodiment of a process of producing the airframe 6 isprovided.

Returning now to the process of producing the jig 60 performed at steps32, FIG. 12 is a process flow chart showing certain steps of anembodiment of a process of producing the jig 60.

At step s50, a plurality of steel beams are attached together, e.g. bywelding, to form a rectangular parallelepiped, thereby producing the jigframe 62. An example jig frame and method of construction thereofincludes, but is not limited to, those described in EP1230124 andEP1600379, each of which is incorporated in its entirety herein byreference. The pick-up devices may be formed from aluminium.

At step s52, the reference frame 64 is attached, e.g. by welding, to oneend of the jig frame 62. In this embodiment, the reference frame 64 is asubstantially exact replica of at least part of the fore fuselage frameto which the airframe 6 of the aft fuselage 4 is to be attached.

At step s54, a CMM measures a surface of the reference frame 64 on thejig 60. In particular, in this embodiment, the CMM measures features ofthe reference frame 64 that correspond to those features of the forefuselage frame to which the airframe 6 is to be attached.

At step s56, using the CMM measurements of the reference frame 64, acomputer determines a datum, herein referred to as the “jig datum”. Thejig datum is a reference system, with respect to the reference frame 64,from which measurements may be made. The jig datum may be computed usingany appropriate software package.

At step s58, a human designer generates or creates a further digitalmodel, hereinafter referred to as the “third digital model”. The thirddigital model is of the airframe and the fore fuselage frame. The thirddigital model specifies the positional relationships between thecomponents of the airframe 6 and the fore fuselage frame, when theairframe 6 is attached to the fore fuselage frame.

The third digital model may be created using a computer and anappropriate software package, for example, the Catia (Trademark) V4software package.

At step s60, using the third digital model, the human designer or acomputer determines positions on the jig frame 60 for each of the jigpickups 66, 68, 70, 72.

In this embodiment, the determined locations on the jig 60 of the firstjig pickups 66 are such that, were the first frame 8 to be held by thefirst jig pickups 66, the position of the first frame 8 relative to thereference frame 64 would be substantially the same as a desired positionfor the first frame 8 relative to the fore fuselage frame onboard theassembled aircraft 2.

Similarly, the determined locations on the jig 60 of the second jigpickups 68 are such that, were the second frame 10 to be held by thesecond jig pickups 68, the position of the second frame 10 relative tothe reference frame 64 would be substantially the same as a desiredposition for the second frame 10 relative to the fore fuselage frameonboard the assembled aircraft 2.

Similarly, the determined locations on the jig 60 of the third jigpickups 70 are such that, were the third frame 12 to be held by thethird jig pickups 70, the position of the third frame 12 relative to thereference frame 64 would be substantially the same as a desired positionfor the third frame 12 relative to the fore fuselage frame onboard theassembled aircraft 2.

Similarly, the determined locations on the jig 60 of the fourth jigpickups 72 are such that, were the second ends of the beams 20, 22 heldby the fourth jig pickups 72, the positions of the second ends of thebeams 20, 22 relative to the reference frame 64 would be substantiallythe same as a desired position for the second ends of the beams 20, 22relative to the fore fuselage frame onboard the assembled aircraft 2.

At step s62, each of the jig pickups 66-72 are attached to the jig frame62, e.g. by bolts or by welding, in the position determined at step s60for that jig pickup.

Steps s64 and s66 are optional steps for verifying and, if necessary,adjusting the position and/or orientation of the jig pickups on the jig60. In some embodiments, these steps are omitted or a differentverification and/or different adjustment process is performed. Steps s64and s66 may be performed iteratively.

At step s64, a laser tracker measures the position of each of the jigreceiving elements 74-80 with respect to the jig datum, i.e. withrespect to the fore reference frame 64.

At step s66, using the laser tracker measurements, the positions of thejig receiving elements 74-80 with respect to the jig datum may bemodified to ensure that each of the jig receiving elements 74-80 has theposition determined at step s60 for that jig receiving element 74-80. Inother words, the positions of the jig receiving elements 74-80 may bemodified to ensure that, when the frames 8, 10, 12 are held by the jigreceiving elements 74-80, the positions of the frames 8, 10, 12 relativeto the reference frame 64 are substantially the same as a desiredposition for the frames 8, 10, 12 relative to the fore fuselage frameonboard the assembled aircraft 2.

Modifying a position of a jig receiving element 74-80 may be performedby, for example, introducing a shim between a jig pickup and the jigframe 62, or by machining part a jig pickup and/or the jig frame 62.

In some embodiments, the positions of one or more of the jig pickups 66,68, 70, 72 measured by the laser tracker may be used to update orrecalculate the jig datum. For example, in some embodiments, a new jigdatum is calculated using measured positions of the reference frame andone or more of the jig receiving elements 74-80. This new jig datum is areference system, with respect to the reference frame 64 and one or moreof the jig receiving elements 74-80, from which measurements may bemade.

Thus, an embodiment of a process of producing the jig 60 is provided.

An advantage provided by the above described methods and apparatus isthat the airframe advantageously is produced within very tight tolerancebounds that tend not to be possible using conventional productiontechniques. The airframe is produced with high accuracy relative to thereference frame which is representative of a forebody frame to which theairframe is to be attached. In particular, the airframe is produced suchthat the IML is within very with tight tolerances with respect to thereference frame. Thus, the assembly of the aircraft, and in particularthe attaching together of the aft fuselage and the fore fuselage, tendsto be facilitated.

Advantageously, the blocks of the frame machining fixture are precisionground with respect to the locator pins of the frame machining fixture(i.e. the frame datum). During production of the frame, each block isused as a “zero point” from which a cutting tool is moved to machine arespective frame longeron landing close to that block. The proximity ofthe block to the associated longeron landing advantageously tends meansthat, in order to form a longeron landing, the cutting tool does nothave to be moved large distances from a “zero point”. Thus, errors tendsto be reduced and the accuracy (with respect to the frame datum) withwhich the longeron landings are formed tends to be increased.

Advantageously, the above described jig tends to use fewer pickups forholding the airframe components compared to conventional assembly jigs.Thus, the weight and cost of the jig tends to be reduced compared toconventional assembly jigs. Furthermore, as airframe components are heldby fewer pickups, damage/stresses resulting from a component being heldby pickups tends to be reduced.

Advantageously, the longeron landings of the frames are machined withhigh accuracy with respect to a local frame datum that is defined byframe features that are used to locate that frame in the assembly jig.Thus, because the jig pickups that couple to the frame attachmentfeatures are accurately located on the jig in a desired position withrespect to the reference frame, when the frames are attached to the jig,the longeron landings are accurately located on the jig in a desiredposition with respect to the reference frame. Thus, when the longeronsare attached to the longeron landings of the frames, the longerons tendto be are accurately located on the jig in a desired position withrespect to the reference frame. Accordingly, gaps or spaces betweenlongerons and frames advantageously tend to be minimised. The use shimsto fill such gaps or spaces tends to be reduced or eliminated.

The above described methods and apparatus tends to provide that theframes of the airframe may be produced independently from one another.In other words, there tends to be no requirement to machine or processthe frames as a set. This independent production of the frames tends toreduce production time of the airframe compared to conventionalprocesses.

A further advantage provided by the above described methods andapparatus is that non-destructive testing and other processes may beperformed on individual airframe components separately. This tends to bedifficult if, in contrast to the above described method, multipleairframe components are machined as a set in an assembly jig.

Apparatus, including the any of the abovementioned computers orprocesses for performing any of the above described data processingmethod steps may be provided by configuring or adapting any suitableapparatus, for example one or more computers or other processingapparatus or processors, and/or providing additional modules. Theapparatus may comprise a computer, a network of computers, or one ormore processors, for implementing instructions and using data, includinginstructions and data in the form of a computer program or plurality ofcomputer programs stored in or on a machine readable storage medium suchas computer memory, a computer disk, ROM, PROM etc., or any combinationof these or other storage media.

It should be noted that certain of the process steps depicted in theflowcharts of FIGS. 4, 7, and 9 and described above may be omitted orsuch process steps may be performed in differing order to that presentedabove and shown in FIGS. 4, 7, and 9. Furthermore, although all theprocess steps have, for convenience and ease of understanding, beendepicted as discrete temporally-sequential steps, nevertheless some ofthe process steps may in fact be performed simultaneously or at leastoverlapping to some extent temporally.

In the above embodiments, an airframe of an aircraft aft fuselage isproduced. However, in other embodiments, a different type of structureis produced, for example a structure that, in use, is to be attached toa different structure may be produced. For example, an airframe of adifferent part of the aircraft, e.g. the fore fuselage, may be produced.In such embodiments, the jig may comprise a different type of referenceframe representing a different type of entity to which the structurebeing assembled is to be attached.

In the above embodiments, the aircraft comprises three frames, threelongerons, two beams, and a plurality of keels. However, in otherembodiments, the airframe comprises a different number of frames,longerons, beams, and/or keels. In some embodiments, one or more of thelisted types of airframe components may be omitted. In some embodiments,the airframe comprises a different type of airframe component instead ofor in addition to those listed above.

In the above embodiments, a frame comprises four attachment featureswhich are used to determine the frame datum. The frame attachmentfeatures are holes through the structure of the frame. Also, the jigcomprises jig pickups configured to attach to the frame attachmentfeatures. Also, the frame machining fixture comprises locator pinsconfigured to couple to the frame attachment features. However, in otherembodiments, one or more of the frames or other airframe componentscomprises a different number of attachment features. In someembodiments, one or more of the attachment features may be a differenttype of attachment feature other than a through hole. Also, the jig maycomprise a different type of pickup that is configured to attach to thedifferent type of attachment feature. For example, in some embodiments,an attachment feature may be a block-like structure and a jig pickup maycomprise a clamp for clamping to the block-like structure. Also, theframe machining fixture may comprise a different type of device that isconfigured to attach to the different type of attachment feature.

In the above embodiments, longeron landings are machined with respect toa local frame datum. However, in other embodiments one or more differenttypes of frame features are formed with respect to the frame datuminstead of or in addition to one or more of the longeron landings.

In the above embodiments, the airframe components are made of aluminium.However, in other embodiments, one or more of the airframe components ismade of a different type of material.

In the above embodiments, the jig frame is a steel beam frame in theshape of a rectangular parallelepiped. However, in other embodiments,the jig frame is made of a different material and/or is a differentshape.

In the above embodiments, the frame machining fixture comprises threeblocks which are used as reference points or surfaces from which a CNCcutting tool is moved. The blocks are located on an upper surface of theframe machining fixture. In other embodiments, the frame machiningfixture comprises a different number of blocks or other devices thatprovide the above described functionality. In some embodiments, one ormore of the blocks is located on a different part of the frame machiningfixture, e.g. on a side of the base portion.

The invention claimed is:
 1. A method of producing a machining fixturefor fixedly holding a workpiece during machining of that workpiece toform a predetermined object, and for machining the predetermined objectfrom the workpiece, the method comprising: providing an initialmachining fixture comprising a plurality of receiving elements forreceiving the workpiece to be machined; determining a datum, the datumbeing dependent upon relative positions of the receiving elements;determining positions and orientations of one or more reference surfacesof the machining fixture with respect to the datum, each of thereference surface of the machining fixture being associated with anobject feature of the predetermined object that is to be produced;measuring a surface of the initial machining fixture with respect to thedatum; and thereafter, controlling machining apparatus with respect tothe datum to machine the initial machining fixture to form the one ormore reference surfaces, thereby producing the machining fixture;providing the workpiece to be machined, the workpiece comprising aplurality of attachment features; attaching the workpiece to themachining fixture by attaching the receiving elements of the machiningfixture to the attachment features of the workpiece; and thereafter,performing, using the machining apparatus a machining operation tomachine the workpiece so as to produce the predetermined object, whereinsaid machining of the workpiece comprises, for each reference surface:locating at least part of the machining apparatus against that referencesurface; and controlling the machining apparatus to move away from thatreference surface with respect to the datum to machine the workpiece soas to form the object feature associated with that reference surface. 2.The method according to claim 1, wherein the machining fixture is anairframe component machining fixture configured for fixedly holding theworkpiece during a process of producing an airframe component from thatworkpiece.
 3. The method according to claim 1, wherein: the methodfurther comprises providing a first digital model of the predeterminedobject to be produced from the workpiece using the machining fixture;and the step of determining positions and orientations of one or morereference surfaces of the machining fixture comprises: using the firstdigital model, identifying one or more features of the object; and foreach identified feature, determining a position and orientation of areference surface of the machining fixture relative to the datum.
 4. Themethod according to claim 3, wherein: the machining fixture is anairframe component machining fixture configured for fixedly holding theworkpiece during a process of producing an airframe component from thatworkpiece; and each object feature is a landing for receiving a furtherairframe component during an airframe assembly process.
 5. The methodaccording to claim 1, wherein the method further comprises, providing afirst digital model of the predetermined object to be produced from theworkpiece using the machining fixture; and using the first digital modeland the determined positions and orientations of the one or morereference surfaces of the machining fixture, determining a seconddigital model, the second digital model being a model of the machiningfixture specifying the positions and orientations of the referencesurfaces of the machining fixture; and the step of controlling machiningapparatus with respect to the datum to machine the initial machiningfixture to form the one or more reference surfaces is performed usingthe second digital model.
 6. The method according to claim 1, whereinthe step of determining the datum comprises: measuring the relativepositions of the receiving elements on the initial machining fixture;and using the measurements of the relative positions of the receivingelements, calculating the datum.
 7. The method according to claim 1,wherein: relative positions of the attachment features of the workpieceto be machined are dependent upon the relative positions of thereceiving elements; and the step of determining the datum comprises:measuring the relative positions of the attachment features on theworkpiece; and using the measurements of the relative positions of theattachment features, calculating the datum.
 8. The method according toclaim 1, wherein the predetermined object is an airframe componentselected from the group of airframe components consisting of a frame anda longeron.
 9. The process according to claim 1, wherein: thepredetermined object is an airframe component selected from the group ofairframe components consisting of a frame and a longeron; and each ofthe object features is a landing for receiving a further airframecomponent during an airframe assembly process.
 10. An apparatus forproducing a machining fixture for fixedly holding a workpiece duringmachining of that workpiece to form a predetermined object, themachining fixture being formed from an initial machining fixturecomprising a plurality of receiving elements for receiving the workpieceto be machined, the workpiece comprising a plurality of attachmentfeatures, the apparatus comprising: one or more processors configuredto: determine a datum, the datum being dependent upon relative positionsof the receiving elements; and determine positions and orientations ofone or more reference surfaces with respect to the datum; measuringapparatus configured to measure a surface of the initial machiningfixture with respect to the datum; and machining apparatus configured tobe controlled with respect to the datum so as to machine the initialmachining fixture to form the one or more reference surfaces, therebyproducing the machining fixture; upon attachment of the workpiece to themachining fixture by attaching the receiving elements of the machiningfixture to the attachment features of the workpiece, the machiningapparatus being further configured to perform a machining operation tomachine the workpiece so as to produce the predetermined object, whereinsaid machining of the workpiece comprises, for each reference surface:locating at least part of the machining apparatus against that referencesurface; and controlling the machining apparatus to move away from thatreference surface with respect to the datum to machine the workpiece soas to form the object feature associated with that reference surface.11. The apparatus of claim 10, wherein the machining fixture comprises arigid base portion, and the receiving elements extend from an uppersurface of the base portion.
 12. The apparatus of claim 11, wherein thereference surfaces are formed from sacrificial blocks attached to thebase portion.